1. Field of the Invention
The present invention relates to an immersion microscope objective, and a microscope using the same.
2. Description of the Related Art
As a means of fluorescence observation, a method of fluorescence observation by multi-photon excitation has been known. In the multi-photon excitation, light of a wavelength in almost integral multiples of an absorption wavelength of a fluorescent substance is irradiated to a fluorescent material (sample) as excitation light. Here, a wavelength of the excitation light in the multi-photon excitation being almost an integral multiple of the absorption wavelength, energy of one photon of the excitation light is almost an integral submultiple for the multi-photon excitation as compared to a single-photon excitation. Therefore, in the multi-photon excitation, it is necessary to make a plurality of photons collide with one fluorescent substance simultaneously.
When the excitation light is converged by a microscope objective, a density of light becomes the highest at a focal position. Therefore, a probability of the plurality of photons colliding with the fluorescent substance becomes the highest at the focal position. In the multi-photon excitation, the fluorescent light is generated only at the focal position (or in the extreme proximity of the focal position). For making the density of light at the focal position high, it is necessary to make a diameter of a light spot formed at the focal position as small as possible. In view of such circumstances, the microscope objective is sought to have a large numerical aperture and a superior imaging performance. If the imaging performance is superior, a light spot in which, various aberrations have been corrected favorably, is formed at the focal position.
Moreover, the excitation light used in the multi-photon excitation is infrared light. Light has a feature of longer the wavelength, lesser is the susceptibility of scattering of light (Rayleigh scattering). Therefore, in a sample having a scattering characteristic such as of a biological sample, the infrared light having a long wavelength is capable of reaching up to even deeper position as excitation light. For observing a deep portion of a sample by using this feature, the microscope objective is sought to have a long working distance.
Furthermore, even among the infrared lights, light having a longer wavelength has smaller Rayleigh scattering. Therefore the excitation light is capable of reaching even deeper position. For this reason, it is preferable that the microscope objective to be used in the multi-photon excitation has a longer wavelength and a superior imaging performance.
Moreover, in the observation of a deep position of a specimen, an aberration due to a refractive index of the sample cannot be ignored. Therefore, it is preferable that the microscope objective has a means of correcting fluctuation in such aberration.
As a microscope objective which has a large numerical aperture and in which, an aberration in an infrared region has been corrected favorably, an immersion microscope objective disclosed in Japanese Patent Application Laid-open Publication No. 2010-008989 is available.